With tubular conduits in which cables are laid, so-called cable ducts, it may be desirable to install in an existing tube a second tube, whose interior has a low(er) coefficient of friction than the existing tube, so that a cable, optionally at a later time, can be pulled through the second tube over a greater length. It may also be desirable to install in an existing tube a larger number of tubes with a smaller cross section than that of the existing tube if it is desired to use each of the smaller tubes as a separate guide or subduct for single- or multi-core copper or glass fiber cables. Further, it may be necessary to install in an existing tube a tube with a water barrier, so that in the existing tube, whose interior gradually fills up with water through diffusion, a waterproof conduit is created by means of the second tube, this waterproof conduit allowing the routing of cables without a water barrier. Such tubes with a water barrier may also have to be installed in existing water supply pipes if it is found that the water in the interior of the existing, slightly porous tube is subject to undue contamination by the surrounding groundwater. Finally, in tubes with a rough inside, a second tube with a smooth inside wall can be installed, so that the flow rate can increase in spite of the decrease of the total cross section. This is already being applied in sewer pipes and can also be applied in water supply pipes or gas or oil transmission lines.
To date, when installing tubes in existing tubes, either use is made of pulling techniques for pulling the new tube or tubes through the existing tube, or the "existing" tube is installed with the second tube or tubes already arranged therein. This last procedure is being applied especially in installing cable ducts having a number of subducts therein.